1. Field of the Invention
The present invention relates to semiconductor devices. More particularly, the present invention relates to a low-voltage punch-through transient suppressor employing a dual base structure.
2. The Prior Art
Electronic circuitry which is designed to operate at supply voltages less than 5 volts are extremely susceptible to damage from overvoltage conditions caused by electrostatic discharge, inductively coupled spikes, or other transient conditions from its operating environment. The current trend of the reduction in circuit operating voltage dictates a corresponding reduction in the maximum voltage that the circuitry can withstand without incurring damage. As operating voltages drop below 5 volts to 3.3 volts and below it becomes necessary to clamp transient voltage excursions to below five volts.
The most widely used device currently in use for low voltage protection is the reversed biased p+n+ zener diode. See O. M. Clark, "Transient voltage suppressor types and application", IEEE Trans Power Electron., vol. 5, pp. 20-26, November 1990. These devices perform well at voltages of 5 volts and above but run into problems when scaled to clamp below 5 volts. The two major drawbacks incurred by using this device structure are very large leakage currents and high capacitance. These detrimental characteristics increase power consumption and restrict operating frequency.
A second device capable low clamping voltages is the n+pn+ uniform base punch through diode, such as disclosed in P. J. Kannam, "Design concepts of high energy punchthrough structures" IEEE Trans. Electron Devices, ED-23, no. 8, pp. 879-882, 1976, and D. de Cogan, "The punch through diode", Microelectronics, vol. 8, no. 2, pp 20-23, 1977. These devices exhibit much improved leakage and capacitance characteristics over the conventional pn diode but suffer from poor clamping characteristics at high currents. If the designer tries to improve clamping to protect circuitry under industry standard surge conditions by increasing die area, the results are devices which are too large to produce economically.
It is therefore an object of the present invention to provide a low-voltage transient suppressor which avoids some of the shortcomings of the prior art.
It is another object of the present invention to provide a low-voltage transient suppressor which has a low leakage current.
It is further object of the present invention to provide a low-voltage transient suppressor which has a lower capacitance than prior-art low-voltage transient suppressors.
It is yet another object of the present invention to provide a low-voltage transient suppressor which has improved high-current clamping characteristics compared to prior-art low-voltage transient suppressors.